Palynostratigraphy of a Jurassic–Cretaceous transitional succession in the Himalayan Tethys,southern Xizang (Tibet), China

A palynological analysis of a Late Jurassic–Early Cretaceous succession in the Himalayan Tethys, Gyangzê County, southern Xizang (Tibet) provides, for the first time, evidence of changing palynofloras through the Jurassic/Cretaceous (J/K) boundary. Species that are stratigraphically important and potential markers for delineating the boundary include both miospores and dinoflagellate cysts. The presence of the spores Crybelosporites sp. cf. stylosus, Foraminisporis wonthaggiensis, Jiaohepollis verus and Toroisporis welzowense and the cysts Cassiculosphaeridia delicata and Rhynchodiniopsis serrata imply that the J/K boundary is between samples 06-21-1 and 06-21-3. The occurrence of Aequitriradites spinulosus and Cicatricosisporites spp. a little below this level and of ?Dictyotosporites sp. cf. speciosus slightly above it is also significant. These results show that it is possible to locate the J/K boundary in the Himalayan Tethys near top of the Weimei Formation and the lower part of the Gyabula Formation in southern Xizang. This succession also contains various marine invertebrate fossils, including ammonites, bivalves and belemnites, and thus has considerable potential for erecting an integrated biostratigraphy around the J/K boundary in the eastern Tethyan realm. Palynofloristic correlation implies a more northerly location for the fossil locality at Gyangzê than that of northwest Australia during the latest Jurassic and earliest Cretaceous, which can be further constrained to around 43°S.     

An Investigation of Deep Electrical Structure of Qiangtang Basin,Xizang(Tibet),China

In order to study the deep geoelectrical structure and the regional geological structure and detect potential oil and gas areas in Qiangtang basin in northern Xizang (Tibet), 222 MT soundings were conducted along three N - S MT profiles across the basin .The MT results indicate that the south and north parts of the Qiangtang basin have a good contrast in the deep electrical structure . In the south Qiangtang , there are generally two high conductivity layers in the crust . The first is at a depth of about 10-25 km and possesses a resistivity of about 10-80 Ωm . The second, the high conductivity layer in the lower crust, is at a depth of about 40-70 km with 3- 50 Ωm . In the north Qiangtang , there is generally one high conductivity layer . It is at a depth of about 10-30 km and the resistivity is about 1-60 Ωm . The thickness of the second high conductivity layer in both the south Qiangtang and the Bangong-Nujiang suture is much greater than that of the first . The thickness of the lithosphere is abou     

Depositional environment and provenance of the upper Permian–Lower Triassic Tianquanshan Formation,northern Tibet: implications for the Palaeozoic evolution of the Southern Qiangtang,Lhasa, and Himalayan terranes in the Tibetan Plateau

Abstract

This article reports the depositional environment and provenance for the Tianquanshan Formation in the Longmuco–Shuanghu–Lancangjiang suture zone, and uses these to better understand the tectonic evolution of this region. Zircons in the andesite of the Tianquanshan Formation yielded concordia ages of 246, 247, and 254 Ma, indicating that the Tianquanshan Formation formed during the late Permian–Early Triassic. The Tianquanshan Formation consists of flysch and ocean island rock assemblages, indicating that the Longmuco–Shuanghu–Lancangjiang Palaeo-Tethys Ocean continued to exist as a mature ocean in the late Permian–Early Triassic. The detrital zircons in the greywackes of the Tianquanshan Formation yielded peak ages of 470–620, 710–830, 910–1080, 1450–1660, and 2400–2650 Ma, indicating the provenance of the Tianquanshan Formation was either Indian Gondwana or terranes that have an affinity with Indian Gondwana in the Tibetan Plateau (i.e. the Southern Qiangtang, Lhasa, and Himalayan terranes). The Ordovician quartzites, Carboniferous sandstones, Carboniferous–Permian diamictites, and the Upper Permian–Lower Triassic greywackes in the Southern Qiangtang, Lhasa, and Himalayan terranes all contain detrital zircons with youngest ages of ca. 470 Ma, indicating their source areas have been in a stable tectonic environment since the Ordovician, and this inference is supported by the continuous deposition in a littoral–neritic passive margin in these regions from the Ordovician to the lower Permian. Combining the present results with regional geological data, we infer that the Southern Qiangtang, Lhasa, and Himalayan terranes were all in a stable passive continental margin along the northern part of Indian Gondwana during the long period from the Ordovician to the early Permian. At early Permian, because of the opening of the Neo-Tethys Ocean, the tectonic framework of this region underwent a marked change to a rifting and active environment.     

Distribution of Hydrogen and Oxygen Isotopes in Salt Lakes of the Qinghai—Xizang Plateau,China

The Qinghai-Xizang Plateau is an area where a large number of salt lakes are distributed. We have collected several hundred samples of natural waters over the Plateau since 1976 and carried out researches on their hydrogen and oxygen isotopes. The results indicate that the^δD and δ¹⁸O values of the salt lake waters over the Plateau range from −64.1 to +12.4‰ and from −11.19 to +8.62‰, respectively. From the different types of surfaces, ground and lake waters of various salinities it is inferred that the compositions of H and O isotopes in the initial water of Qinghai Lake are^δD=−55.0‰ and {ie336-1}; and those in the original water from the lakes in northern Xizang, are^δD=−116.0‰ and {ie336-2}. Brines in the salt lakes are derived from rain water through prolonged circulation. Oilfield water also makes some contribution to the salt lakes in the Qaidam Basin. Similar slopes of evaporation lines of water isotopes are noticed for the Qinghai Lake area and northern Xizang. This is attributed to the evolution of the isotopes in these water bodies in an environment of middle latitude and high elevation.     

The Characteristics of Yongzhu–Guomang Lake Ophiolitic Melange in Bangong-Nujiang Suture, Xizang(Tibet)，China

正Yongzhu–Guomang Lake ophiolitic melange exposed about 100 km with large scale and complete ophiolitic uint in Xainza County,Xizang(Tibet).It is connected with Nam Lake,Kaimeng ophiolitic mélange to the east,and     

Petrogeochemistral Characteristics of the Acid Intrusive Rocks in the Zhaxikang Deposit, Xizang（Tibet）

Please refer to the attachment(s) for more details.     

Ordovician Proto–basin in South China and its Tectonic Implications: Evidence from the Detrital Zircon U–Pb Ages of the Ordovician in Central Hunan,China

Caledonian orogeny is another important tectonic event in South China Block after the breakup of the Rodinia supercontinent. With a view to constrain the tectonic evolution and proto–basin in South China, this paper reports the geochemical and zircon U-Pb dating data of the Ordovician strata in central Hunan, South China. Geochemical features and paleocurrent directions suggest that the lower Ordovician deposited in a passive continental margin basin with a provenance of quartzose components and...     

Micro faunal assemblages in the Ordovician – Silurian successions from the Pin Valley of Tethys Himalaya, India

The Spiti basin together with the Zanskar basin forms the largest basin among the Tethyan Himalayan successions and forms one of the best-developed sec-tions in the Tethyan Tibetan belt. The basin is one of the classical areas, which depicts a continuous fos-siliferous Palaeozoic - Mesozoic successions. The present studies are focused on the Ordovician and Si-lurian successions of the Pin valley of the Spiti basin. Pin valley exposes richly fossiliferous lithological successions from Neoproterozoic to Cretaceous; therefore, it is an ideal section for the detail paleobi-ological and geological studies.     

Opx–Cpx exsolution textures in lherzolites of the Cretaceous Purang Ophiolite (S. Tibet,China), and the deep mantle origin of Neotethyan abyssal peridotites

ABSTRACT

We investigated lherzolitic peridotites in the Cretaceous Purang ophiolite along the Yarlung Zhangbo suture zone (YZSZ) in SW Tibet to constrain their mantle–melt evolution history. Coarse-grained Purang lherzolites contain orthopyroxene (Opx) and olivine (Ol) porphyroclasts with embayments filled by small olivine (Ol) neoblasts. Both clinopyroxene (Cpx) and Opx display exsolution textures represented by lamellae structures. Opx exsolution (Opx1) in clinopyroxene (Cpx1) is made of enstatite, whose compositions (Al₂O₃ = 3.85–4.90 wt%, CaO = <3.77 wt%, Cr₂O₃ = 0.85–3.82 wt%) are characteristic of abyssal peridotites. Host clinopyroxenes (Cpx1) have higher Mg#s and Na₂O, with lower TiO₂ and Al₂O₃ contents than Cpx2 exsolution lamellae in Opx, and show variable LREE patterns. Pyroxene compositions of the lherzolites indicate 10–15% partial melting of a fertile mantle protolith. P–T estimates (1.3–2.3 GPa, 745–1067°C) and the trace element chemistry of pyroxenes with exsolution textures suggest crystallization depths of ~75 km in the upper mantle, where the original pyroxenes became decomposed, forming exsolved structures. Further upwelling of lherzolites into shallow depths in the mantle resulted in crystal–plastic deformation of the exsolved pyroxenes. Combined with the occurrence of microdiamond and ultrahigh-pressure (UHP) mineral inclusions in chromites of the Purang peridotites, the pyroxene exsolution textures reported here confirm a multi-stage partial melting history of the Purang lherzolites and at least three discrete stages of P-T conditions in the course of their upwelling through the mantle during their intra-oceanic evolution.     

Gas Occurrence and Accumulation Characteristics of Cambrian–Ordovician Shales in the Tarim Basin, Northwest China

The Tarim Basin is located in northwestern China and is the biggest basin in China with huge oil and gas resources. Especially the Lower to Middle Cambrian and Middle to Upper Ordovician possess the major marine source rocks in the Tarim Basin and have large shale gas resource potential. The Cambrian–Ordovician shales were mainly deposited in basin–slope facies with thicknesses between 30–180 m. For shales buried shallower than 4500 m, there is high organic matter abundance with TOC (total organic carbon) mainly between 1.0% and 6.0%, favorable organic matter of Type I and Type II, and high thermal maturity with RoE as 1.3%–2.75%. The mineral composition of these Cambrian–Ordovician shale samples is mainly quartz and carbonate minerals while the clay minerals content is mostly lower than 30%, because these samples include siliceous and calcareous shale and marlstone. The Cambrian and Ordovician shales are compacted with mean porosity of 4% and 3%, permeability of 0.0003×10?3–0.09×10?3 μm2 and 0.0002×10?3–0.11×10?3 μm2, and density of 2.30 g/m3 and 2.55 g/m3, respectively. The pores in the shale samples show good connectivity and are mainly mesopore in size. Different genetic types of pores can be observed such as intercrystal, intergranular, dissolved, organic matter and shrinkage joint. The reservoir bed properties are controlled by mineral composition and diagenesis. The maximum adsorption amount to methane of these shales is 1.15–7.36 cm3/g, with main affecting factors being organic matter abundance, porosity and thermal maturity. The accumulation characteristics of natural gas within these shales are jointly controlled by sedimentation, diagenesis, hydrocarbon generation conditions?, reservoir bed properties and the occurrence process of natural gas. The natural gas underwent short-distance migration and accumulation, in-place accumulation in the early stage, and adjustment and modification in the later stage. Finally, the Yulin (well Y1) and Tazhong (well T1) areas are identified as the targets for shale gas exploration in the Tarim Basin.     

Cyclostratigraphic Calibration of the Upper Ordovician (Sandbian–Katian) Pagoda and Linhsiang Formations in the Yichang Area, South China

正1 Introduction The Sandbian-Katian is a critical period for the transition from"hot-house"in the Lower Ordovician to"ice-house"in the end-Ordovician (Trotter et al.,2008).During this interval,the South China Block was located in the equatorial region (Torsvik and Cocks,2016),with the widespread accumulation of Pagoda and Linhsiang formations (Zhan and Jin,2007).Although these strata have been investigated for decades and their     

Carbonate diagenesis of the upper Jurassic successions in the west of Binalud — Eastern Alborz (NE Iran)

The upper Jurassic carbonate settings in Iran are widely exposed in north and northeastern parts. Five stratigraphic columns were selected in the north eastern Iran. Their thickness ranges from 330 to 500 m. The various diagenetic processes identified include, micritization, cementation, compaction (physical and chemical), dissolution, neomorphism, pyritization, hematitization, silicification and dolomitization, which affected these carbonates. Elemental and stable isotopes analysis indicated that these deposits have undergone both meteoric and burial diagenesis in a relatively open system with moderate water-rock interaction. The positive trend between trace elements and oxygen isotope depletion also support these burial conditions. Lighter δ^l8O values of the dolomite samples may be related to an increase in temperature during the burial, which correspond to coarser euhedral crystals. Relatively higher δ¹⁸O values in finer dolomite crystals indicate their formation at lower burial depths relative to coarser crystals. Petrographic evidences such as coarse euhedral crystals with bright and dull zonation prove this interpretation. Chert nodules also have lighter ¹⁸O values relative to carbonate host rock, thus indicating the influence of burial diagenetic processes in their formation. The average environmental palaeotemperature was estimated to be 26°C on the basis of oxygen isotope values of less altered lime-mudstones.     

Geochronology and Geochemistry of Ore-related Granitoids in the Giant Gariatong Rb Deposit, Tibet and Implications for Rb Metallogeny in China

Rubidium (Rb) deposits mostly occur in the South China and Central Asia orogenic belts and are often closely associated with highly differentiated granites. This study investigates a newly-discovered giant Rb deposit at Gariatong in the Central Lhasa terrane in Tibet. Detailed field studies and logging data revealed that the Rb mineralization mainly occurs in monzogranite and is related to greisenization. LA-ICP-MS U-Pb dating of zircon yielded ages of 19.1 ± 0.2 Ma and 19.0 ± 0.2 Ma for greisenized monzogranite and fresh monzogranite, respectively. The monzogranites are characterized as strongly peraluminous, with high contents of SiO2, Al2O3, K2O and Na2O as well as a high differentiation index. They are enriched in light rare earth and large ion lithophile elements with signi?cant negative Eu anomalies and depleted high field-strength elements. Petrological and geochemical features of these ore-related monzogranites suggest that they are highly fractionated S-type granites, derived from remelting of crustal materials in a post-collisional setting. The geochemistry of zircon and apatite points to a low oxygen fugacity of the ore-related monzogranite during the magma’s evolution. The discovery of the Gariatong Rb deposit suggests that the Central Lhasa terrane may be an important region for rare metal mineralization.     

K-bentonite,black-shale and flysch successions at the Ordovician–Silurian transition,South China: Possible sedimentary responses to the accretion of Cathaysia to the Yangtze Block and its implications for the evolution of Gondwana

The K-bentonite, black shale and flysch successions at the Ordovician–Silurian transition in South China have been the subject of comprehensive investigations relative to the probable accretion of the Yangtze Block and the questionable Cathaysia Block. First, the geochemical analyses of K-bentonites show that the parent magma originated in syn-collisional, volcanic-arc and within-plate tectonic settings, which produced mainly intermediate-to-felsic series magmas, associated with continuous collision and subduction of paleo-continental blocks/arcs. Further, the regional distribution of K-bentonite thickness indicates that voluminous explosive volcanism was located in the present southeastern shoreline provinces of China. Secondly, northwestwardly migrating, Ordovician–Silurian, transitional flysch successions, and the accompanying diachronous K-bentonite-bearing black-shale interval, as well as the related, overlying, shallowing-upward succession at the interior of the Yangtze Block, developed as an unconformity-bound sequence that mirrors foreland-basin tectophase cycles in the Appalachian basin. The above features suggest that the sequence accumulated in a similar foreland basin, which formed in response to adjacent deformational loading in a northwesterly migrating orogen located to the southeast. Geochemical and paleocurrent data from the turbiditic flyschoid sandstones also support these depositional settings. Accordingly, it seems that all criteria strongly support the presence of an Ordovician–Silurian, subduction-related orogen resulting from collision with a block to the southeast that must have been the original “Cathaysia Block” of Grabau and later workers. The K-bentonite, black-shale and flysch successions can be regarded as distal, foreland responses to the continuous northwestward collision and accretion of the Cathaysia Block to the Yangtze Block. Hence, we prefer to suggest that the suture zone with the sensu stricto Cathaysia Block probably developed along previously identified late Early Paleozoic suture relicts in the shoreline provinces of southeast China. On the other hand, although accretion of fragments with Cathaysian affinities to the Yangtze Block may have begun as early as Middle to Late Proterozoic time, the Ordovician–Silurian orogeny described above probably reflects the final phase of accretion between the two blocks. Moreover, when combined with similar peri-Iapetan orogenic events in other areas during the same period, this accretion event may have been part of a major stage of global tectonic reconstruction in the evolution of Gondwana.     

Evolution of Stromatoporoidea in the Ordovician–Silurian epicontinental basin of the Siberian Platform and Taimyr

The paper discusses the evolution of Stromatoporoidea in the epicontinental sedimentary basin of the Siberian Platform and Taimyr during the Ordovician and Silurian. Specimens of the oldest genus, Priscastroma, were found in the middle of Middle Ordovician sediments. This genus is represented by the species P. gemina Khrom., which has two forms, A and B. Tracing the emergence of new genera over time, we identified two distinct branches in stromatoporoid evolution.The ancestor of the first branch is P. gemina f. A, which gave rise to the genus Cystostroma. The latter is the ancestor of two subbranches with predominant horizontal skeletal elements. The subbranches differ only in tissue microstructure. The genera Stromatocerium, Dermatostroma, and Aulacera display dense fibrous microstructure, whereas the genus Rosenella and its descendants display dense microstructure. The genus Lophiostroma, with a lamellar–fibrous tissue, may be a dead branch of evolution.The ancestor of the second branch is P. gemina f. B, which gave rise to the genus Labechia and its descendants. This branch has a dense tissue, with predominant vertical skeletal elements.Ordovician stromatoporoids from Siberia were compared with those from other basins of the world. Comparison shows that all the Ordovician genera from the epicontinental basin of the Siberian Platform and Taimyr originated here. Thus, this basin was one of the centers of stromatoporoid origin.     

Remarks on sequence stratigraphy and taphonomy of the Malvinokaffric shelly fauna during the KAČÁK Event in the Apucarana Sub-basin (Paraná Basin), Brazil

The Eifelian–Givetian (Middle Devonian) transition constituted an important paleoenvironmental event for the Malvinokaffric Realm in the Apucarana Sub-basin (Paraná Basin). This study highlights integration between taphonomy and sequence stratigraphy, and four depositional sequences are identified during the transition. In Sequence 1, the presence of a typical normal-sized Malvinokaffric fauna is recorded. In the transgressive systems tract (TST) of Sequence 2, no benthic fossils are present, and this is interpreted as a stratigraphic marker of an event of significant paleoenvironmental change (KA?ÁK Event). In Sequence 3, the TST has abundant bioclasts, which become rarer toward the top of the section, i.e., within the transgressive systems tract. The original habitat of this autochthonous to parautochthonous fauna was a low-energy environment between the fair weather wave base and the storm wave base of the Devonian epicontinental sea. In the highstand systems tract of this sequence, the presence of normal-sized Pennaia paulianna and lingulids demonstrates the return of more ambient conditions. Sequence 4 is of Carboniferous age. Its limit is a second-order sequence boundary recording a lowstand systems tract formed by a fluvial depositional system. The low diversity and the disappearance of taxa are not the result of a taphonomic bias, but reflect the post-KA?ÁK Event.     

Conodonts from the Cambrian–Ordovician boundary interval in the southeast margin of the Sichuan Basin,China

Conodonts from the Cambrian–Ordovician transition at the Liangcun section in Xishui County, Guizhou and at the Huangcao section in Wulong County, Chongqing are examined for the first time. Both sections are located at the southeast margin of the Sichuan Basin. A total of 1367 specimens were recovered, representing 30 species and 15 genera. Based on the ranges of conodonts generalized from these two sections and another six sections previously studied in the same region, three conodont zones, Cordylodus proavus, Monocostodus sevierensis and Cordylodus angulatus zones are recognized. The index species of the Cambrian–Ordovician boundary at the global stratotype section and point (GSSP), Iapetognathus fluctivagus and its substitute in China Iapetognathus jilinensis are not observed in the study sections, therefore it is impossible to determine the Cambrian–Ordovician boundary exactly. However, it probably lies within the lower part of M. sevierensis zone (the upper part of the Loushanguan Group), correlating with the GSSP in Canada and the Dayangcha section in China. Chronological sequences of the FAD (First Appearance Datum) of C. angulatus, Chosonodina herfurthi and Rossodus manitouensis are not obvious in the study, so the C. angulatus zone here is correlated with zones defined by C. angulatus, Ch. herfurthi and R. manitouensis in the lower Yangtze Platform.